Fuel cells, which are small devices but realize highly-efficient electric power generation, have been developed as electric power generating systems of distributed energy supply sources. However, means for supplying a hydrogen gas necessary as a fuel for electric power generation is not developed as a general infrastructure. Therefore, a hydrogen generator configured to generate a hydrogen-containing gas by utilizing a raw material gas, such as a city gas or a propane gas, supplied from an existing raw material gas infrastructure and causing a reforming reaction between the raw material gas and water may be attached to the electric power generating system.
The hydrogen generator is typically configured to include: a reformer configured to cause the reforming reaction between the raw material gas and the water; a shift converter configured to cause a water gas shift reaction between carbon monoxide and steam; and a selective oxidizer configured to oxidize the carbon monoxide mainly by an oxidizing agent, such as a small amount of air. In these reactors, catalysts suitable for the respective reactions are used. For example, a Ru catalyst or a Ni catalyst is used in the reformer, a Cu—Zn catalyst is used in the shift converter, and a Ru catalyst or the like is used in the selective oxidizer. Each of the reactors has an appropriate temperature. Typically, the reformer is used at about 600 to 700° C., the shift converter is used at about 200 to 350° C., and the selective oxidizer is used at about 100 to 200° C. Especially, since an electrode of a polymer electrolyte fuel cell tends to be poisoned by CO, the CO concentration of the supplied hydrogen-containing gas needs to be kept to several tens of volume ppm. Therefore, the CO concentration needs to be reduced by oxidizing the CO in the selective oxidizer.
Here, the raw material gas, such as a city gas, contains a sulfur compound. Since the sulfur compound is a poisoning material of especially a reforming catalyst, it has to be removed in some way. Proposed are a hydrogen generator which adopts as a sulfur compound removing method a method for removing the sulfur compound by normal temperature adsorption (see PTL 1, for example) and a hydrogen generator which adopts as the sulfur compound removing method a method for removing the sulfur compound by hydrodesulfurization using the recycled hydrogen-containing gas (see PTL 2, for example). Since the normal temperature adsorption does not require hydrogen, handling thereof is easy, but the adsorption capacity thereof is not large. Since the hydrodesulfurization requires heating and hydrogen, handling thereof is not easy, but the adsorption capacity thereof is large. Here, also proposed is a hydrogen generator which includes and uses both a normal temperature absorbent desulfurizer and a hydro-desulfurizer (see PTLs 2 and 3, for example).